1. Field of the Invention
This invention relates to sodium sulphur cells and to cathode current collectors for such cells and methods of making such cells and current collectors.
2. Prior Art
In a sodium sulphur cell, a solid electrolyte material, typically beta alumina, separates molten sodium, constituting the anode, from the cathodic reactant comprising molten sulphur and sodium polysulphides. The cathodic reactant is highly corrosive. The cathode current collectors must make electrical contact with the cathodic reactant and many proposals have been made regarding the construction of such current collectors. Although carbon and graphite are widely used because of their corrosion-resistant properties, they do have a very substantially higher electrical resistance than most metals. For this reason, use is made of composite current collectors having a substrate of a relatively highly conductive material with an outer sheath or coating of some material having good corrosion-resistant properties. Such an arrangement enables an aluminium substrate to be employed giving the advantages of the high conductivity of that material; aluminium however if exposed to the cathodic reactant in a sodium sulphur cell will form a sulphide coating which is non-conductive. It is important therefore that the outer coating should protect the aluminium from any exposure to the cathodic reactant. Reference is made to U.S. Pat. Nos. 3,982,975; 4,024,320; 4,061,840 and 4,219,690 as examples of composite cathode current collectors for sodium sulphur cells.